The present invention relates to a gas turbine combustor and, more particularly, to a gas turbine combustor which is partly of diffusion type and partly of premixed type.
In general, exhaust emissions from a gas turbine combustor contain substances such as NOx, CO, UHC (unburnt hydrocarbon) and the like which are causes of air pollution. Among these noxious substances, NOx is under a strict control by regulations, and various methods have been proposed for reducing emission of NOx. For example, in a method known as wet-type NOx generation reducing method, water or steam is sprayed into a combustor. This method, however, is inevitably accompanied by a reduction in the efficiency of the gas turbine plant. Therefore, in recent years, a method known as dry-type NOx generation reducing method is becoming a major method. In this method, fuel is supplied into a combustor in a plurality of stages so that the combustion load in each stage is reduced and the combustion is conducted with a lean mixture so as to avoid generation of any local hot spots, thus reducing generation of NOx. However, it is not allowed to employ a large number of combustion stages, due to restriction in the design and construction. In general, therefore, so-called two-staged combustors having fuel nozzles at upstream and downstream sides of the combustor are widely used. This type of combustor is disclosed, for example, in Japanese Patent Unexamined Publication No. 56-25622. FIG. 2 shows an example of such a two-staged combustor. Referring to this Figure, the combustor has a cylindrical combustion sleeve 7 which is divided into two sections in the longitudinal direction thereof so as to define a sub-chamber 1 which is formed in an upstream side (left-hand side in FIG. 2) and intended for a first-stage combustion and a main chamber 2 which is formed in a downstream side and intended for a second-stage combustion.
A first-stage fuel nozzle 3 is disposed in a region near the upstream end of the sub-chamber 1. This fuel nozzle 3 is of a diffusion type combustion in which the fuel 30 injected therefrom is diffused in the surrounding air c so as to be combusted with the air c.
An air swirler 4 is disposed at the end 22 of the main chamber 2 of the second stage. The fuel 32 discharged from a second-stage fuel nozzle 5 is premixed with air n within the air swirler 4 so as to perform premixed combustion at the outlet of the air swirler 4.
FIG. 3 is a graph showing the characteristics of generation of NOx both in the diffusion combustion and premixed combustion, wherein the abscissa represents the fuel-to-air ratio and the ordinate represents the NOx content in relative value.
The stoichiometric fuel-to-air ratio for methane gas is 0.058 and the gas turbine combustor is usually operated with a fuel-to-air ratio which is below this stoichiometric value. In single-staged combustors which have been conventionally used in countries where there is no requirement for reduction of NOx emissions, the fuel-to-air ratio at rated operation was in a range of around 0.04. Since the diffusion combustion is partly employed in this system, the NOx characteristic of this system exhibits a tendency such that the change in relative value of NOx content is comparatively small even if the fuel-to-air ratio is changed. Further, as shown in FIG. 3, in the premixed combustion it is a characteristic that the relative value of NOx content is drastically reduced when the fuel-to-air ratio decreases. The conventional combustor designed for reduced NOx emission is a so-called hybrid combustor in which these diffusion and premixed combustions are combined and, in operation thereof, the diffusion combustion alone is used while the load level is still low after the start-up of the gas turbine and the premixed combustion and the diffusion combustion are simultaneously used in the load range between a light load and a rated load. In FIG. 3, the flame in the sub-chamber 1 is the diffusion combustion flame, while the flame in the main chamber 2 is the premixed combustion flame. Conventional hybrid combustor employs an internal air flow rate control mechanism as indicated by IFC in FIG. 3 for the purpose of controlling the fuel-to-air ratio, in order to expand the region of the premixed combustion flame in the main chamber. In the past, regulations relating NOx reduction were not so strict, so that a simple two-staged combustion with a mere addition of premixed combustion zone (fuel flame F.sub.2) could satisfy such regulations. In recent years, however, the regulations relating to NOx reduction are becoming stricter and, to meet such a demand, it has become necessary to effectively reduce generation of NOx also in the first stage, i.e., in the diffusion combustion zone (fuel flame F.sub.1). To this end, it has been attempted to reduce generation of NOx by reducing the fuel-to-air ratio in the first stage combustion zone (fuel flame F.sub.1) and providing the sub-chamber 1 with an inner sleeve 10 (FIG. 2) so as to expand the diffusion combustion flame and thus increase a contact area of the flame with the air to thereby reduce the flame temperature.
This arrangement, however, presents a new problem relating to the difficulty in maintaining the diffusion combustion flame. FIG. 4 graphically illustrates a characteristic of change in the fuel-to-air ratio relative to the rotational speed of the gas turbine and the level of the load on the gas turbine. More specifically, in FIG. 4, the abscissa represents the rotational speed and the load level, while the ordinate represents the fuel-to-air ratio. Ignition of the fuel is effect when the gas turbine rotational speed is 16% of the rated speed and acceleration of the gas turbine commences after a period of warm-up which usually continues several minutes from the ignition. The fuel-to-air ratio drastically increases and reaches a peak during the acceleration. The fuel-to-air ratio is then progressively decreased and is minimized when the gas turbine has been accelerated to the rated speed under no load.
The fuel-to-air ratio then starts to increase again in accordance with an increase in the load level and is maximized immediately before the rated load is reached. The fuel nozzle 3 for the fuel flame F.sub.1 (FIG. 2) is used from the start-up of the gas turbine until the load increases to about 30% of the load under the rated speed, and injects a large amount of the fuel during acceleration of the gas turbine. When a large amount of the fuel is injected, a shortage in oxygen takes place in the region around the fuel nozzle 3 because the space is restricted in this region, so that the flame from the fuel nozzle 3 tends to flameout towards the main chamber having a comparatively large space thereby resulting in the flame being extinguished as a result of contact with cold air. The rotational speed of the gas turbine at which the blow off of the flame takes place has a certain relationship to the temperature of the air discharged from the gas turbine and, the flameout or flame extinguishing is retarded as the air temperature is raised.
FIG. 5 graphically illustrates a relationship between fuel flow rate (ordinate) and gas turbine rotation speed (abscissa). In FIG. 5, a gas turbine operation curve represents the state of operation of the gas turbine as determined by the gas turbine rotational speed and the fuel flow rate. A mark X on this curve represents a flameout point. In the illustrated case, the flameout takes place when the rotation speed is comparatively low. A curve representing the upper limit against the flameout is obtained by connecting a plurality of flameout points. Similarly, there is a curve representing the lower limit against blow off of the flame. Therefore, in the start-up of the gas turbine it is necessary that the gas turbine combustor be operated under conditions which fall within the range between the curves representing the upper and lower flameout limits but this makes it impossible to increase an incremental ratio of the fuel flow rate, so that it takes the fifteen to twenty minutes for the gas turbine to be started and reach the rated speed under no load.
Thus, while the conventional constructions mentioned above are effective for reducing the NOx generation, with respect to the quick start-up of the gas turbine, the conventional approach is disadvantageous in that, during the start-up of the turbine, the flameout takes place or a very long time is required until the rated rotational speed is reached although there is no flameout.